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Interstitial Fluid (ISF - The Internal Sea for Life)


Contents

Body Fluids
Extracellular Fluids (ECF)
Interstitial Fluid (ISF)
Cell Transport
Oedema
Hydrostatics Property of ISF

Body Fluids

Some theories on the "Origin of Life" suggest that life arose from the sea. Specific environment varies according to different scenario. It could be in hydrothermal mounds or tidal pool (see "f" and "e" in Figure 01). While composition of the ancient sea 4 billion years ago could be different from the present; very recent sample shows that it is not much different from the interstitial fluid bathing all cells in multicellular organisms, i.e., it is salty with lot of Na+ and Cl- ions (Figure 02). The importance of interstitial fluid was observed back to 1887 by Claude Bernard. Its role has been
Origin of Life Sea Water and ISF reinforced in modern time by such similarity. While unicelluar organisms can acquire nutrients and expel waste directly in aquatic environment, they could not survive on dry surfaces with a humidity of less than 10%. The same situation is applicable to the cells in multicelluar organisms which solve the problem with the "Internal Sea" in the form of Interstitial Fluid (ISF).

Figure 01 Origin of Life
[view large image]


Figure 02 Sea Water and Body Fluids,
Composition [view large image]


Figure 02 also shows that the fluid composition inside the cell (Intracellular Fluid, ICF) is quite different from sea water as the constituents are altered by the metabolic process of life.

In human body, about 60% (in weight) is fluid on average. It varies between female and male, and also depends on age from 100% in fetus to about
Body Fluids 40% in the elderly (Figure 03). There are two kinds of body fluids, namely, the intracellular (ICF) and extracellular (ECF) which is further categorized into plasma and interstitial fluid (ISF). Since the source of ISF is derived from the plasma, they have essentially the same composition with lower concentration of proteins in ISF (Figure 02). In addition, small amount of lymph in the lymphatic system and sometimes TSF (Transcellular Fluids such as cerebrospinal, ocular and joint fluids ) are also included into the ECF in some accounting. The composition of ICF varies depending on the type of organ. As illustrated in Figure 03, the lungs contains 90% water while the bones are very dry at 22%.

Figure 03 Body Fluids
[view large image]

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Extracellular Fluids (ECF)

As shown in Figure 04 (left), there are four types of tissue, which are groups of cells that have a similar shape and function. They are the epithelium, muscle, nervous, and connective. Different types of tissues can be found in different organs. Connective tissue maintains the form of organs throughout the body. It provides a matrix that supports and physically connects other tissues and cells together in organs. This extracellular matrix (ECM) includes the basement membrane [sheet-like layer secreted by the epi(on)-, endo(within)-, thelial(layer) cells on which it sits], and fibrous
Connective Tissue and Extracellular Matrix (ECM) proteins called interstitial matrix. It acts as a compression buffer against the stress placed on the ECM as shown in Figure 04 (right). The interstitial fluid (ISF) is another component that fills the rest of the ECM space providing a vital service for the maintenance of life. Nutrient and waste from and to the ISF must diffuse across the basement membrane or through gaps (for the discontinuous type, see Figure 05) to be absorbed by the cells.

Figure 04 Connective Tissue and (ECM) Extracellular Matrix [view large image]


As shown in Figure 06, the ISF is part of the extracellular fluid, which includes the plasma. The plasma is the carrier to deliver nutrient (collected from the lungs and small intestine) and to drain waste (back to the lungs and excretory system). It is the heart that provides the pressure to move the plasma to the arteries - capillaries - veins. The capillaries (Figure 05) are the sites where nutrient passes to the ISF and waste back at the other end.
Types of Capillaries Global View of ECF The exchange is facilitated through the difference between the hydrostatic and osmotic pressures (see Figure 06, top). About 2% of the ISF is drained via the lymphatic system, which depends on the body movements to force the fluid into the lymphatic capillaries. All the cells in multicellular organisms obtain nutrient and remove waste through the ISF - the "Internal Sea" (serum in medical term).

Figure 05 Types of Capillaries [view large image]

Figure 06 Global View of ECF [view large image]

BTW, red blood cells and platelets cannot pass through the walls of the capillaries (too large).

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Interstitial Fluid (ISF)

Closer examination of the ISF reveals that it contains a mixture of nutrient and waste indicating a primitive origin of the organization. This property is a hallmark of sea water which contains all kind of substances whether it is useful or useless or even harmful to the living organisms. The selection and rejection are performed by concentration gradient or osmosis pressure. It is very inefficient similar to the three chambers heart of the amphibian, in which the deoxygenated and oxygenated blood are mixed in the ventricle before being pumped out of the heart (see more in "Different Hearts").

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Cell Transport

Cell Transport The cells in various organs are at the other end of ISF running the reverse process (for capillary) of receiving nutrient and rejecting waste. There are many methods according to the kind of substance. Transport of smaller molecules depends on concentration gradient as shown in Figure 08a. Figure 08b provides a global view of the salt and water transport through the cellular membrane. The larger molecules are transported via a more elaborate process (Figure 09).

Figure 08 Cell Transport [view large image]

The three types of cellular transport for smaller molecules are explained briefly below.
In addition, macro-molecules such as some food particles have to be swallowed by endocytosis, in which the substance is enclosed by a vesicle and drawn inside. The reverse process is called exocytosis, while transcytosis refers to movement of the engulfing vesicle within the cell (Figure 09). These processes require energy to execute and actually belong to the active transport category.

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Oedema

Oedema Edema is an abnormal accumulation of fluid in ISF manifesting as swelling. The amount of ISF is determined by the balance of fluid homeostasis, which controls the proportion of plasma and ISF in the ECF to a range of (20% - 25%) and (80% - 75%) respectively. The increased secretion of fluid into ISF, or the impaired removal of the fluid, can cause the condition. The symptoms vary according to the location where the swelling occur (see article by "Mayo Clinic"). Figure 10 shows a case of

Figure 10 Oedema [view large image]

leg swelling caused by blocking or loss of the lymphatic capillary.

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Hydrostatics Property of ISF

Unlike blood flow along the arteries and veins forced by the action of the heart pump, the interstitial fluid (ISF) is static in nature. It follows the principle of hydrostatics, from which the hydraulic action is derived. Accordingly, ISF can transmit hydrostatic pressure from one point to another. An example is the action of the needle in acupuncture, which generates fluid pressure or pushing/shearing force to clear up any blockage (Figure 11).
Acupuncture Hydrostatic Pressure Transmission While the effect of mechanical force on embryonic development has been demonstrated conclusively by many experiments. The most common examples of hydrostatic pressure transmission are the various kinds of massage, which facilitate the circulation of interstitial fluid by mechanical force. Meditation helps to promote recycling of the fluid by relaxing the muscles as well as the mind. Qi-gang directs the pressure to move along a direction (need supervision to prevent blockage or bleeding through wrong pathway). Then some forms of martial art use the internal fluid pressure against external force (Figure 12).

Figure 11 Acupuncture

Figure 12 Hydrostatic Pressure Transmission, Effect of

See also ""Mechano-transduction".